This invention relates generally to a bearing for a horizontally oriented rotatable shaft, and in particular to a hybrid bearing which incorporates the desirable characteristics of a journal bearing and a tilting pad bearing.
It is well known in the art that fixed arc, cylindrical journal bearings have low power loss characteristics and are relatively easy to manufacture. However, these fixed arc journal bearings exhibit relatively poor dynamic characteristics. In particular, these journal bearings are susceptible to a form of self excited vibration known as "oil whirl" caused by cross coupling stiffness characteristics of the bearing. As is well recognized, a film of oil flows between the rotating shaft and the inner surface of the fixed arc journal bearing. Cross coupling stiffness characteristics of the bearing result from the fact that the vector forces generated in this oil film by displacements of the rotating shaft are not in line with the shaft displacements.
Tilting pad bearings, which include a plurality of pads circumferentially placed about the shaft and pivotably mounted within the bearing casing, minimize the cross coupling vector forces contributing to the "oil whirl" phenomenon. The radially outer surface of each tilting pad normally has a smaller radius of curvature than that of the inner surface of the bearing casing thereby allowing the pad to pivot or tilt in response to movement by the shaft. Since the pad can pivot, it follows that the net oil film force for each pad must be centered over the pivot. This tends to result in oil film forces which are in line with shaft displacements i.e., tends to minimize cross coupling forces. However, tilting pad bearings do experience a relatively high power loss, in the range of 25-30% higher than the power loss experienced by fixed arc, cylindrical journal bearings. Also, tilting pad bearings are relatively more expensive to manufacture than the cylindrical journal bearings.
When the shaft is relatively horizontally oriented, the upper, unloaded tilting pads may rock excessively or pivot which causes fretting or wear at the pivotal contact portions of the radially outer surface of the tilting pad and/or the adjacent portions of the inner surface of the bearing casing. An article in Power Engineering, on November, 1982, discloses what appears to be a four pad tilting bearing. A close review of the article supports this description of FIG. 4, on page 64, which shows the end casing above the horizontal section line and two tilting pad bearings as a cutaway view below the horizontal section line.